Advances in biotechnology have made it possible to produce a variety of proteins for pharmaceutical applications using recombinant DNA techniques. Because proteins tend to be larger and more complex than traditional organic and inorganic drugs, the formulation of such proteins poses special problems. For a protein to remain biologically active, a formulation must preserve the conformational integrity of at least a core sequence of the protein's amino acids, while at the same time protecting the protein's multiple functional groups from degradation. Degradation pathways for proteins can involve chemical instability (i.e. any process which involves modification of the protein by bond formation or cleavage resulting in a new chemical entity) or physical instability (i.e. changes in the higher order structure of the protein). Chemical instability can result from, for example, deamidation, racemization, hydrolysis, oxidation, beta elimination or disulfide exchange. Physical instability can result from, for example, denaturation, aggregation, precipitation or adsorption. Three common protein degradation pathways are protein aggregation, deamidation and oxidation (Cleland et al. Critical Reviews in Therapeutic Drug Carrier Systems 10(4): 307-377 (1993)).
Freeze-drying is a commonly employed technique for preserving proteins which serves to remove water from the protein preparation of interest. Freeze-drying, or lyophilization, is a process by which the material to be dried is first frozen and then the ice or frozen solvent is removed by sublimation in a vacuum environment. An excipient may be included in pre-lyophilized formulations to enhance stability during the freeze-drying process and/or to improve stability of the lyophilized product upon storage (Pikal, M. Biopharm. 3(9)26-30 (1990) and Arakawa et al. Pharm. Res. 8(3):285-291 (1991)).
Therefore, the need still exists for developing protein formulations, particularly for subcutaneous administration, that are stable for long-term storage and delivery.